December 30, 2004: Get ready for two of the strangest hours in the history of space exploration.

Two hours. That's how long it will take the European Space Agency's Huygens probe to parachute to the surface of Titan on January 14th. Descending through thick orange clouds, Huygens will taste Titan’s atmosphere, measure its wind and rain, listen for alien sounds and, when the clouds part, start taking pictures.

No one knows what the photos will reveal. Icy mountains? Liquid methane seas? Hot lightning? "It's anyone's guess," says Jonathan Lunine, a professor of planetary science at the University of Arizona and a member of the Huygens science team. "We might not even understand what we see, not immediately."

Astronomers have been watching Titan, Saturn's largest moon, for centuries. From Earth it looks like a pinprick of light orbiting the ringed planet—nothing extraordinary. But when NASA's Voyager spacecraft flew by Titan in 1980, observers realized it was something special. Titan is huge: bigger than the planets Mercury and Pluto. It has a huge atmosphere, too: three times taller than Earth's and one and a half times as massive. The air on Titan is choked with organic compounds akin to smog. Some of these molecules are building blocks of life. Could life begin on a world where the surface temperature dips 290o F below zero? "Probably not," says Lunine, but, again, no one knows.

Titan's orange clouds hide its surface and, maybe, some pretty bizarre things. There's methane (CH4) in Titan's atmosphere. Here on Earth methane comes from, e.g., cows and bogs. On Titan … no one knows where it comes from. Because Titan is so cold, its methane can liquefy and rain down from the skies possibly filling lakes and seas on the ground. Liquid methane has about the same appearance and viscosity as ordinary water, but it's some 300o F colder. Lakes on Titan, if they exist, might look like lakes on Earth, but they certainly won't be the same.

Above: This false-color view of Titan is a composite of images captured by Cassini's infrared camera, which can penetrate some of Titan's clouds. Light and dark regions in the upper left quadrant are unknown types of terrain on Titan's surface. [More]

The Huygens probe, about the size of a small car and shaped like a flying saucer, will penetrate the clouds and investigate first hand. "We're so hopeful that Huygens will succeed," says Alfred McEwen, a colleague of Lunine's at the University of Arizona and a member of the Cassini imaging team. "We're pulling our hair out trying to understand Titan."

Huygens rode to Saturn onboard NASA’s Cassini spacecraft. The trip lasted 7 years. Cassini arrived in July 2004 and is now orbiting Saturn. Huygens remained onboard until Dec. 25th when it separated from its mothership and headed for Titan. The probe is scheduled to enter Titan’s atmosphere at 10:13 GMT (5:13 a.m. EST) on January 14, 2005.

On the way down, Huygens will collect air samples for analysis by onboard gas chromatographs and mass spectrometers. This will tell researchers exactly what Titan's atmosphere is made of. Huygens' external sensors will measure temperature, pressure, winds and electromagnetic fields that might come from lightning. Lightning is important. Seething-hot strokes can fuse simple organic molecules into more complicated and interesting things. Some scientists think this is how life began on Earth billions of years ago. A microphone onboard Huygens will listen for thunder (a sign of lightning) and other sounds. For the first time, we'll get to hear what another world sounds like.

Huygens is going to descend during daylight hours. Sunlight filtering through the clouds probably casts an orange glow across the landscape "like 1000 full moons," says McEwen. That's bright enough to read a newspaper, but still about 1000 times dimmer than a sunlit day on Earth. Just before Huygens lands it will turn on an intense flashlight and shine it on the terrain below. This is done to improve pictures of the landing site and help the probe's spectrometers get better readings of elements and minerals in the soil--or whatever's down there.

"We don't know what we’re going to land on," notes Lunine. Huygens might go tumbling down a cliff. It might splash into a lake or sea (Huygens is designed to float). Or it might thump down on a smooth icy plain. "Just about anything is possible."

Right: An artists' concept of Huygens floating in a liquid methane sea. [More]

If the probe survives touchdown, the Surface Science Package attached the bottom of the saucer can measure the properties of the landing site: thermal and electrical conductivity, index of refraction, sonar depth and many other things. Mission planners hope Huygens survives on the "ground" for at least 30 minutes before Titan’s bitter cold and unknown hazards shut it down. Even a few minutes of data would be cause for celebration.

While all this is happening, the Cassini spacecraft will be flying overhead, recording Huygens' transmissions. Later, Cassini will turn toward home and relay the pictures and sounds and priceless measurements. Radio signals from Saturn take 1 hour and 8 minutes to reach Earth. "We can't wait to get the data," says McEwen.

What's down there? No one knows, but it's bound to be strange. Get ready.